Process for converting scrap tires to rubber particles while removing inorganic compounds from the tires

ABSTRACT

Tires are converted to rubber particles and inorganic compounds are removed from the tires by treating the tires in a closed reactor with a nitric acid solution having a nitric acid concentration above 60%. In the preferred embodiment, any steel belts contained in the tires are first removed. The tires are then treated with a solution of concentrated nitric acid in which the acid concentration is maintained above 60% which extracts inorganic compounds such as zinc, sulfur and calcium from the tires and converts the tires into rubber particles.

BACKGROUND OF THE INVENTION

The present invention relates to processes for recycling scrap tires.More particularly, the present invention relates to processes fortreating scrap tires to remove inorganic components, steel belts and toconvert the tires to rubber particles.

Currently, about 200 million scrap tires are being stockpiled,landfilled or illegally dumped annually with about 50 million scraptires being recycled in a variety of ways. The current disposal methodsare causing numerous short term environmental problems. For example,whole tires occupy large amounts of space and may "float" or rise to thetop of landfills. In an attempt to prevent floating, many landfillsrequire that scrap tires be shredded, a process which is energyintensive and wasteful if it does not produce any useful product.

Scrap tire stockpiles produce health risks by providing a place forrodents and mosquitoes to breed which facilitates the spreading ofdiseases. Large fires have also broken out in scrap tire stockpilescausing many problems. These fires are long lasting and are difficult toextinguish, unnecessarily tying up fire fighting resources.Additionally, these fires produce unwanted smoke which pollutes theenvironment and toxic oils which poison adjacent soils and water.

Accordingly, numerous attempts have been made to develop processes touse or recycle scrap tires.

Scrap tires often contain several inorganic components such as zinc,sulfur and calcium which are impregnated into the rubbery portion of thetire. These inorganic components can be a hindrance to the recycling ofthe rubbery portion of the tires using various processes. For example,their removal is necessary when the tires are to be combusted in aconventional boiler since the inorganic components will cause airpollution and/or boiler scale build-up as well as interfering withvarious catalytic processes.

Many tires also include steel belts and bead wires for added strengthand support. These belts and wires also must be removed from the scraptires in order to permit effective recycling and use of the rubberyportion of the tire.

Accordingly, it would be a significant advancement in the art to providea process for removing steel belts and other inorganic components fromscrap tires to permit more effective recycling of the rubbery portion ofthe tires. It would be a further advancement to provide a process thatconverted tires into small particles for subsequent use. Such a processis disclosed and claimed herein.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a novel process for removing inorganiccomponents and steel belts from scrap tires while converting the tiresto particulates. In a preferred embodiment, whole tires and/or tirepieces are mixed with a nitric acid solution in which the nitric acidconcentration is maintained above 60% in a closed adiabatic system tocleave the bonding that adheres rubbery polymer in the tire pieces toany steel belts that are contained in the tires. These steel belts canthen be removed by any suitable means such as with a magnet.

The rubbery tire pieces are then further processed with a nitric acidsolution with a nitric acid concentration above 60% in a closedadiabatic system to extract inorganic compounds embedded within therubber. After the rubbery components have soaked for a sufficient timesuch that the inorganic components have been extracted, the rubberypieces are separated from the nitric acid solution by filtration. Thefiltrate is then washed, dried and powdered.

This process removes inorganic components such as zinc, sulfur andcalcium from the tires, allows recovery of the steel belts and convertsthe tires to rubber particles which can be used for other purposes.

In the preferred embodiment, the nitric acid concentration in theprocessing solutions is monitored and maintained above 60% by theaddition of gaseous nitrogen dioxide and oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram illustrating a preferred embodimentof the process of the present invention.

FIG. 2 is a schematic illustration of a reactor useful in a preferredembodiment of the process of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a novel process for converting scraptires to particulates while removing steel belts and unwanted inorganiccomponents to leave a rubber composition which can more easily berecycled. The invention is best understood by reference to the attacheddrawings.

Reference is made to FIG. 1 which schematically illustrates a preferredembodiment of the process of the present invention. Scrap tires 10 arepreferably prepared for processing by removing dirt and foreignmaterials. The tires can be treated whole or they can be shredded or cutinto smaller pieces to facilitate processing in the processes of thepresent invention.

In a preferred embodiment, the scrap tire pieces are first subjected toa process 12 for steel belt removal. The tires are mixed in a closedadiabatic system with a concentrated nitric acid solution 14 having anacid concentration greater than 60% which cleaves the bonding thatadheres the rubbery polymer of the tire pieces to the steel belts. Inone embodiment, the nitric acid solution begins with an acidconcentration of about 70%. After the tires have soaked in the nitricacid solution for a sufficient length of time to cleave the bonds, thesteel belts 16 can be removed by a magnet or other suitable means. Inthe preferred embodiment, the tires are soaked in the nitric acidsolution at ambient temperature. At about 22° C., it takes about 11/2 to3 days to separate the steel belts from the tire particles.

In most steel belted tires, the belts are connected to the rubber by acopper-sulfur bond. The NO₃ ⁻ (aq) ion attacks and cleaves this bond,thus separating the steel belts from the rubber. Thus, it is importantto maintain a high nitric acid concentration in the solution.

The nitric acid concentration of the solution is maintained above 60%during this process by the addition of gaseous nitrogen dioxide andoxygen. The oxygen can be added to the closed system either as air orpure oxygen. The specific gravity of the solution is monitored and thenitrogen dioxide and oxygen are added to the vapor phase as needed. A60% nitric acid solution has a specific gravity of about 1.37 g/cc. Thenitrogen dioxide and oxygen gases react in solution to replenish theHNO₃ (aq) and NO₃ ⁻ (aq) by the following reactions:

    4NO.sub.2 (g)+O.sub.2 (g)→2N.sub.2 O.sub.5 (g)

    N.sub.2 O.sub.5 (g)+H.sub.2 O→2HNO.sub.3 (aq)

    HNO.sub.3 (aq)→H.sup.+ (aq)+NO.sub.3.sup.- (aq)

It is the balance of the solution phase species HNO₃ and NO₃ ⁻ at verylow pH that is important to the process of the present invention.

While the above described process is the preferred method to remove thesteel belts from the scrap tires, other processes can also be used. Forexample, the steel belts can be mechanically removed. Also, sulfuricacid and hydrogen peroxide solutions can be used to separate the beltsfrom the rubber particles.

The rubber pieces remaining after steel belt removal are introduced intoa closed extraction reactor 22 where they are mixed with a nitric acidsolution 24 having a nitric acid concentration above 60% to removeinorganic components embedded within the rubber pieces. The nitric acidsolution degrades the tire pieces into particles and dissolves theinorganic components such as zinc, sulfur and calcium contained therein.The process is preferably performed adiabatically. It will beappreciated that when nitric acid is used both as the solvent for steelbelt removal and for inorganic extraction, it is not necessary to removethe nitric acid by filtration step 18. Rather, the steel belt removaland inorganic extraction can take place in the same reactor.

The rubber pieces are soaked in the nitric acid solution for about 4-5days at ambient temperature to remove the in organics. During this time,the specific gravity of the solution is monitored and gaseous nitrogendioxide and oxygen are added as necessary to maintain the nitric acidconcentration above 60%. If nitric acid is used to remove the steelbelts that treatment time is included in the total treatment time toremove the in organics.

Sulfur in the tires is removed by the following reaction in which Rrepresents the rubber:

    R-S+8H.sup.+ (aq)+8NO.sub.3.sup.- (aq)→R+SO.sub.4.sup.-2 (aq)+8NO.sub.2 (g)+4H.sub.2 O

This results in a very large production of NO₂ (g). Accordingly, inorder to maintain the NO₃ ⁻ (aq) concentration at a high enough level inthe solution, oxygen must be added to the system. In the preferredembodiment this is accomplished by adding air to the vapor phase of theclosed system.

After the inorganics have been extracted from the rubber, the solvent 28which includes the inorganic compounds is separated from the rubberparticles 30 by a suitable filtration step 26.

If nitric acid is used to cleave the bonds holding the steel belts, thesame solution can be used for that process as well as the process forthe extraction of the inorganics. It is, however, preferred to removethe steel belts as soon as they can be separated to conserve the nitricacid which would otherwise be consumed in dissolving the belts.

The processed rubber 30 forms a pulpy material which can easily bewashed, dried and ground into particles. These particles can be recycledthrough a number of different processes. For example, since they arebasically carbon and hydrogen and have a high heat content, they are anexcellent pollutant-free fuel source producing a heat value of about 32Joules/gram. They can be used in particle form or can be compressed intopellets using standard processes.

They can also be mixed with other fuels such as rice hulls, municipalgarbage or low rank coals to produce synthetic blends that are useful asfuels. In one preferred embodiment, the processed rubber particles aremixed with sawdust and/or wood chips to form a fuel having about thesame specific heat as bituminous coal. Such a mixture producessignificantly less sulfur dioxide in the effluent and significantly lessash than typical coal.

The rubber particles produced by the process of the present inventioncan be used to extract some metal ions and some anions from water. Forexample, the rubber particles can be used to remove Cd(II) and Cl⁻ froma solution containing CdCl₂.

Reference is next made to FIG. 2 which schematically illustrates aclosed reactor 40 which can be used in the process of the presentinvention. Reactor 40 includes a tank 42 into which a plurality of tires44 can be placed. Tank 42 is then filled with a solution of nitric acid46 through inlet 47 above the level of the tires 44.

A probe 48 is used to monitor the specific gravity of solution 46.Gaseous nitrogen dioxide and oxygen are added to the vapor phase abovesolution 46 through inlets 50 and 52 as needed to maintain the nitricacid concentration above about 60% in solution 46.

After an initial processing period, the steel belts can be removed fromtank 42. After complete processing, the nitric acid solution 46 and therubber particles from tires 44 can be removed through outlet 54. Therubber can then be filtered out, washed, dried and further processed.The nitric acid solution can be recycled or processed for disposal.

EXAMPLE 1

Tires containing steel belts are immersed in a solution of nitric acidin a closed reactor at room temperature and are allowed to soak forabout 36 hours. The specific gravity of the solution is monitored andthe concentration of nitric acid in the solution is maintained above 60%by the addition of gaseous nitrogen dioxide and oxygen (added as air) tothe vapor phase in the reactor as needed. The tires are then removedfrom the solution and the steel belts are removed with the applicationof some mechanical force.

The tires are again immersed in a solution of nitric acid in a closedreactor at room temperature and are allowed to soak for three additionaldays. Again, the nitric acid concentration is maintained above 60%. Atthis time the tires have broken down into particles and they are removedfrom the solution by filtration. Analysis of the rubber particles showsthat substantially all of the sulfur, calcium and zinc have been removedfrom the rubber particles.

EXAMPLE 2

Tires containing steel belts are immersed in a solution of concentratednitric acid in a closed reactor and are allowed to soak at roomtemperature. The concentration of the nitric acid is maintained above60% by the addition of gaseous nitrogen dioxide and oxygen as needed.After about 72 hours, the steel belts in the tires are completelyseparated from the rubber particles and can be removed with a magnet orby other means. The rubber particles are allowed to continue soaking inthe nitric acid for about 2 additional days. The particles are thenseparated from the solution by filtration. Analysis shows thatsubstantially all of the sulfur, calcium and zinc have been removed fromthe rubber particles.

While the invention has been described with respect to the presentlypreferred embodiments, it will be appreciated by those skilled in theart that it can be embodied in other forms without departing from itsspirit or essential characteristics. For example, other processes forremoving the steel belts can also be utilized. Additionally, the lengthof time the tires are allowed to soak can be increased or decreased asnecessary to remove the steel belts and inorganic compounds present inthe particular tires being processed. Accordingly, all modifications orchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A process for converting scrap tires to rubberparticles comprising:mixing tire pieces in a closed reactor with anitric acid solution having a nitric acid concentration above 60% for atime sufficient to extract inorganic compounds from said tire pieces andto break said tire pieces into rubber particles; maintaining theconcentration of the nitric acid above 60% bv the addition of gaseousnitrogen dioxide and oxygen; and separating the solution and extractedinorganic compounds from the rubber particles.
 2. A process forconverting scrap tires to rubber particles as defined in claim 1 whereinthe oxygen is added as air.
 3. A process for converting scrap tires torubber particles as defined in claim 1 wherein the maintaining stepfurther comprises monitoring the specific gravity of the solution.
 4. Aprocess for converting scrap tires to rubber particles as defined inclaim 1 wherein said mixing is performed at ambient temperatures forabout 4 days.
 5. A process for converting scrap tires to rubberparticles as defined in claim 1 wherein the inorganic compounds comprisezinc, sulfur and calcium.
 6. A process for converting scrap tires torubber particles as defined in claim 1 wherein said separation stepcomprises filtering the rubber particles from the solution.
 7. A processfor converting scrap tires to rubber particles as defined in claim 1further comprising removing any steel belts from the tire pieces priorto mixing the tire pieces with the nitric acid solution.
 8. A processfor converting scrap tires to rubber particles as defined in claim 7wherein the removing step comprises mixing the tire pieces with a nitricacid solution to cleave the bonding that adheres the rubber particles ofthe tire pieces to the steel belts.
 9. A process for converting scraptires to rubber particles as defined in claim 8 further comprisingcollecting the steel belts with a magnet.